Previous studies in adults with chronic renal failure (CRF) suggested that reduction of dietary protein may retard the rate of decrease of GFR in such patients. In this application, a multicenter collaborative study will assess the effects of two levels of dietary protein in infants with CRF. The long-term objective of the study is to determine the optimal dietary protein intake in such infants, in terms of both maintenance of GFR and linear growth rate. Specific questions to be posed are as follows: 1) Does reduction of dietary protein intake reduce the rate of deterioration of GFR? 2) Will the rate of linear growth be impaired in infants treated with the low protein formula? The questions will be answered by comparing GFR and growth in response to two levels of protein intake in infants with chronic renal insufficiency (i.e. residual function 12.5 to 50% normal [SCr .8-3.2 mg/dl]); these are infants considered to be at risk for growth retardation and progressive loss of residual renal function. The study will enroll infants under 6 months (mo) of age who have SCr within the stated ranges and meet other criteria. These infants will be stabilized with respect to electrolyte, mineral and calorie intake up to 8 mo of age on a low phosphorus formula plus selected infant foods providing 1.8-2.2 g/kg/day protein, 90-100 kcal/kg and vitamin D supplementation. At 8 mo they will be randomized to a LOW (1.3-1.9 g/kg) or a CONTROL (2.1-2.7 gm/kg/day) protein formula + diet. After a 2-mo period of observation to reduce the risks of protein deficiency in the LOW or protein toxicity in the CONTROL protein group, growth in length and change in GFR over 8 mo will be compared in the two groups from 10-18 mo of age. Growth in length will be measured using a standardized technique and GFR will be measured by "true" creatinine clearance and clearance of non-radioactive iothalamate using a continuous infusion technique. In addition, we will evaluate child development and parent-child interaction, and will perform tests for amino acid imbalance based on fasting and post-feeding amino acid blood levels, and for abnormalities in growth factors (IGF-I, II) to determine the relationship between these variables and growth failure in the infants under study. It is hoped that the information that is obtained from our proposed study may prevent or delay the subsequent development of end-stage renal disease in such patients while allowing normal growth and development.